1. Field of the Invention
The present invention generally relates to power supply systems and voltage converters and, more particularly, to resonant DC/DC converters and power supplies having four or more resonating elements to achieve both increased efficiency and other desired characteristics such as start-up and short-circuit over-current protection.
2. Description of the Prior Art
Many currently available and foreseeable commercial electronic products provide for operation from DC power sources such as rechargeable batteries but also provide for obtaining power from commonly available AC distribution systems. Many such devices include digital logic circuitry which may be of a wide range of complexity and which may include technologically advanced components with stringent tolerances for voltage and current requirements including widely varying current demands at very low voltages such as are encountered in providing power to microprocessors. At the same time, there is a growing demand for both energy savings/high efficiency and simplicity and size reduction for economy of manufacture and convenience when placed in service.
To achieve high efficiency, switching converters have been employed as an alternative to analog voltage regulator circuits. To achieve high power density (e.g. the capability to deliver high power per unit volume) with switching converters, it has been found most effective to increase the switching frequency so that the size of passive components such as filter capacitor and inductor elements may be reduced since such element occupy a major portion of the volume of a switching converter of a given design and electrical specifications.
So-called hard-switching pulse width modulating (PWM) converters essentially regulate voltage for varying current load by altering the duty cycle of input of a generally higher voltage to a converter which may be of any of a number of known topologies functioning as a filter and can achieve efficiencies well above those of analog voltage regulator circuits which regulate voltage by developing a voltage drop across them, thus dissipating substantial power. However, the efficiency which can be achieved by hard-switching converters is limited by the switching losses which become especially severe as switching frequencies are increased.
So-called soft switching PWM converters such as a full bridge phase shift PWM converter or asymmetrical half bridge PWM converter are widely applied for front-end DC/DC conversion. Soft switching PWM converters which include a resonant circuit which is resonant near the switching frequency can achieve zero-voltage-switching (ZVS) since the resonant circuit reduces the voltage to zero or near-zero at the times switching must occur. Therefore, lower switching loss and higher frequency switching with improved efficiency can be accomplished compared with hard switching converters. However, the resonant circuit causes large currents to circulate within the converter, particularly at harmonics of the switching frequency, and conduction losses can be significant. Moreover, holdup time specifications require the converter to maintain output voltage more than 20 mS after AC input line drops. Therefore, bulky capacitors are used to provide energy during the holdup time. Thus the capacity of the holdup time capacitor is wholly determined by required energy during holdup time and severely limits the capability for increasing increased power density even though increasing the operating range can extract more power from the holdup capacitor and allow a capacitor of somewhat reduce value to be employed. Unfortunately, sacrifice of a significant degree of efficiency is required to extend the operating range in order to do so through known techniques. While some arrangements have been proposed for extending operational range and extending holdup time for a given capacitance, they are complex and difficult to control.
LLC resonant converters have also been proposed and have been extensively studied. While LLC converters can achieve zero voltage switching (ZVS) and zero current switching (ZCS) together with high voltage gain capability suitable for reducing the holdup capacitance requirement, currents during start up and short circuit conditions excessively stresses internal components, compromising reliability. Further, the magnitude of circulating resonant currents is aggravated; imposing an increased limitation of efficiency.